1. Field of the Invention
The present invention relates to non-volatile semiconductor devices for storing information using non-volatile variable resistance elements, and more specifically to a semiconductor memory device which allows efficient writing and erasing of data and a control method for the semiconductor memory device.
2. Description of the Related Art
In recent years, there have been proposed high-speed operable next-generation non-volatile random access memory (NVRAM) devices that might replace flash memories and that have various device structures, such as a ferroelectric RAM (FeRAM), a magnetic RAM (MRAM), and an ovonic unified memory (OUM), and there is fierce development competition to increase performance and reliability, reduce cost, and enhance process consistency. However, the current memory devices described above have their merits and demerits, and the developers are facing many difficulties in realizing ideal “universal memory” combining the respective benefits of static RAM (SRAM), dynamic RAM (DRAM), and flash memory.
Instead of the existing technologies described above, resistive non-volatile memory (or resistive random access memory (RRAM (registered trademark))) devices including a variable resistance element having an electrical resistance that reversibly changes in response to the application of a voltage pulse have been proposed. The structure of the variable resistance element is very simple. As indicated in an example illustrated in FIG. 8, a variable resistance element 10 has a structure in which a lower electrode 11, a variable resistor 12, and an upper electrode 13 are stacked in this order from bottom to top and in which an electrical stress such as a voltage pulse is applied between the upper electrode 13 and the lower electrode 11 to provide reversible changes in resistance value. A resistance value in the reversible resistance changing operation (hereinafter referred to as a “switching operation,” if necessary) is read, thereby making a novel non-volatile memory device feasible.
The variable resistor 12 may be formed of a perovskite material known for having a colossal magnetoresistance effect, and a method for applying a voltage pulse to a perovskite material to reversibly change the electrical resistance is disclosed in U.S. Pat. No. 6,204,139, issued to Shangquing Liu, Alex Ignatiev, et al., University of Houston, Houston, Tex. (US), and by Baek, I. G., et al., in “Highly Scalable Non-volatile Resistive Memory using Simple Binary Oxide Driven by Asymmetric Unipolar Voltage Pulses,” IEDM2004, pp. 587-590, 2004. In the element structure illustrated by way of example in U.S. Pat. No. 6,204,139, the variable resistor 12 is formed of a film of crystalline praseodymium calcium manganese oxide Pr1−XCaXMnO3 (PCMO), which is a perovskite-type oxide. In addition, as known from the above-described non-patent literature, films of transition metal oxides, namely, hafnium oxide (HfO2), titanium oxide (TiO2), nickel oxide (NiO), zinc oxide (ZnO), and niobium oxide (Nb2O5), also exhibit a reversible resistance change.
The use of, particularly, a binary metal oxide facilitates miniaturization since a material available in the existing semiconductor production line can be used, and therefore provides an advantage of low-cost manufacturing. In order to implement a desired switching operation through the use of such binary metal oxide, a thin metal oxide film is sandwiched between metal electrodes, and the structure of the variable resistance element is made asymmetric such that an ohmic junction or the like is formed at the interface between one of the metal electrodes and the oxide film and a state where gaps occur for conductive carriers, for example, a Schottky junction, is formed at the interface between the other metal electrode and the oxide film. The structure described above allows the variable resistance element to exhibit a transition between a high-resistance state and a low-resistance state in response to the application of voltage pulses having different polarities, thereby implementing desired bipolar switching.
The non-volatile semiconductor memory device described above includes a memory cell array including a plurality of memory cells arranged in a matrix in the row direction and the column direction, each memory cell including a variable resistance element, and a peripheral circuit that controls the writing, erasing, and reading of data with respect to the respective memory cells in the memory cell array. Examples of the memory cells include, depending on their composition, a memory cell (referred to as a “1T-1R memory cell”) including one selection transistor T and one variable resistance element R, a memory cell (referred to as a “1D-1R memory cell”) including one diode D and one variable resistance element R, and a memory cell (referred to as a “1R memory cell”) including one variable resistance element R.
It is said that a variable resistance element including binary metal oxide, such as hafnium oxide, described above, undergoes resistance switching in accordance with the opening and closing of a conductive path (hereinafter referred to as a “filament path”, if necessary) of oxygen defects, which is in the form of a filament in an oxide film. The filament path is formed as a result of a soft breakdown by the limitation of electric current during dielectric breakdown through the application of voltage called forming. Therefore, the smaller the thickness of the filament path is, the smaller the amount of electric current required for the opening and closing of the filament path which causes the change in resistance, that is, the amount of electric current required for the resistance switching, is.
In order to switch the variable resistance element described above to the high-resistance state, a voltage pulse is applied so that the electrode with a larger work function among the two electrodes has a positive polarity. Thus, the oxygen ions in the film diffuse toward the electrode having a larger work function due to the internal electric field, and Joule heat is generated by the electric current flowing through the filament path, resulting in the migration of oxygen ions in the oxygen defects formed by diffusion or the migration of oxygen ions in the oxide layer around the filament into the filament path. As a result, it is considered that the oxygen defects of the filament disappear and the resistance of the filament increases.
In the case of switching to the low-resistance state, in contrast, a voltage pulse is applied so that the electrode with a larger work function has a negative polarity, thereby generating oxygen defects in the filament path. In this case, the electric current flowing in the variable resistance element is limited by a transistor or the like, thereby forming a low-resistance stable filament path. The lower limit of the electric current required for the opening and closing of the filament path is generally reduced to approximately 100 μA to 200 μA.
In actuality, the variable resistance element including metal oxide described above is applied to a large-capacity semiconductor memory device, by using state-of-the-art miniaturization and processing technology. To this end, it is desirable that data held in the variable resistance element be rewritten or read with the drive capability of a miniature transistor produced using state-of-the-art processing.
In a transistor produced using state-of-the-art miniaturization and processing technology, the on-resistance of the transistor increases, the electric current driving capability decreases, and the driving voltage decreases in accordance with the scaling law. There are currently demands for changes in the resistance state of the element under a writing condition of a voltage as low as approximately 1 V and an electric current as low as several tens of microamperes, and there is also a demand for a further reduction in the electric current required for the opening and closing of the filament path. This implies that, assuming that an electric current of 10 μA flows when a voltage of 1 V is applied in order to change the resistance from the low-resistance state to the high-resistance state, the resistance value in the low-resistance state needs to be set to 100 kΩ if a voltage-current characteristic in the low-resistance state is linear.
Furthermore, the resistance state of the variable resistance element is read by reading the resistance value while the transistor and the variable resistance element are connected in series. For this reason, the reading sensitivity is impaired and difficulties occur in reading unless the resistance value of the variable resistance element in the low-resistance state is set sufficiently higher than the on-resistance of the transistor. However, the higher the resistance value of the variable resistance element in the low-resistance state, the smaller the read margin between the resistance value of the variable resistance element in the low-resistance state and the resistance value thereof in the high-resistance state. For example, assuming that the lower limit value of the reading current that can be sensed using a sense amplifier is 1 μA, the resistance value in the low-resistance state has an upper limit of 100 kΩ if the reading voltage is 0.1 V. The on-resistance of the series-connected transistor is difficult to read unless the on-resistance is set lower than at least the upper limit of the resistance value of the variable resistance element in the low-resistance state.
In the operation of rewriting the variable resistance element from the high-resistance state to the low-resistance state (hereinafter referred to as the “set operation”, if necessary), a voltage of a certain value or more and an electric current of a certain value or more, which are enough to break the bonds between metal and oxygen in the metal oxide, are applied to induce the movement of oxygen in the manner described above, and oxygen vacancies are formed through the metal oxide to form a filament path. In this case, a reduction in the amount of voltage and electric current applied in the set operation or the time during which electric current flows may cause incomplete movement of oxygen, resulting in a discontinuous filament path being formed. This increases the resistance. This mechanism is similar to that in a soft dielectric breakdown (hereinafter referred to as a “soft breakdown”) of a silicon oxide film of several nanometers. It is considered that a thin oxide film composed of metal and oxygen still have the mechanism described above although the amount of electric current required for the set operation, and the time during which electric current flows differ depending on the metal oxide material.
FIG. 9 illustrates a relationship between the pulse width (“set time”) of a voltage pulse applied in the set operation and the resistance value obtained after the set operation in a memory element including a variable resistance element formed of metal oxide, namely, hafnium oxide, and a transistor.
In a case where an electric current (“set current”) Iset flowing through the variable resistance element in the set operation is limited to 100 μA or less using a transistor, a resistance value of approximately 20 kΩ after the set operation is obtained even though the set time is reduced to 100 ns. As the set current is limited to a lower electric current, the resistance value after the set operation is shifted to the high-resistance side. In a case where the set current Iset is limited to 40 μA or less, the resistance value after the set operation tends to increase in accordance with a decrease in the set time, and the resistance value after the set operation increases up to approximately 300 kΩ with respect to a set time of 500 ns or less. Similarly, in a case where the set current Iset is limited to 20 μA or less, the resistance value after the set operation increases up to approximately 10 MΩ with respect to a set time of 500 ns or less.
In a case where the set current Iset is larger than 100 μA, in contrast, the resistance value after the set operation is kept substantially constant regardless of the set time. It is found that the larger the voltage amplitude of the voltage pulse, the shorter the set time.
This implies that, taking into account the drive capability of a small transistor, a set time as long as 1 μs or more may be required to stably control the resistance value after the set operation at approximately 100 kΩ with a set current of several tens of microamperes.
On the other hand, the operation of rewriting the variable resistance element from the low-resistance state to the high-resistance state (hereinafter referred to as the “reset operation”, if necessary) does not cause a reduction in operation speed. As described above, the reset operation is caused by the Joule heat generated by the passage of an electric current through the filament path whose resistance has been converted to a low resistance. Thus, the smaller the set current is, the narrower the filament path to be formed is, resulting in concentration of the Joule heat generated in the filament path. Accordingly, desired Joule heat may be obtained with a smaller amount of electric current. For example, in the element including hafnium oxide, described above, the resistance of a filament path in a low-resistance state of 100 kΩ is converted to a high resistance up to several tens of mega ohms to several hundreds of mega ohms by the passage of an electric current of several tens of microamperes through the filament path for a time of several tens of nanoseconds.
As a result, the feature that has been perceived as an advantage of a resistance-change memory, in which it is possible to perform rewriting in response to the application of a high-speed pulse voltage with random access for several tens of nanosecond, is lost in a large-capacity semiconductor memory device suitable for state-of-the-art miniaturization and processing technology. Such a semiconductor memory device takes a time of 1 μs or longer to perform the set operation, resulting in a reduction in rewriting speed.